Ion trap mass spectrometers have conventionally operated with a three-dimensional (3D) quadrupole field formed, for example, using a ring electrode and two end caps. In this configuration, the minimum of the potential energy well created by the radio-frequency (RF) field distribution is positioned in the center of the ring. Because the kinetic energy of ions injected into an ion trap decreases in collisions with buffer gas molecules, usually helium, the injected ions naturally localize at the minimum of the potential well. As has been shown using laser tomography imaging, the ions in these conventionally constructed ion traps congregate in a substantially spherical distribution, which is typically smaller than about 1 millimeter in diameter. The result is a degradation of performance of the device when attempting to trap large numbers of ions, due to space charge effects.
As one possible solution to this problem, quadrupole mass spectrometers having a two-dimensional quadrupole electric field were introduced in order to expand the ion storage area from a small sphere into an extended cylindrical column. An example of this type of spectrometer is provided in U.S. Pat. No. 5,420,425 to Bier, et al. The Bier, et al. patent discloses a substantially quadrupole ion trap mass spectrometer with an enlarged or elongated ion occupied volume. The ion trap has a space charge limit that is proportional to the length of the device. After collision relaxation, ions occupy an extended region coinciding with the axis of the device. The Bier, et al. patent discloses a two-dimensional ion trap, which can be straight, or of a circular or curved shape, and also an ellipsoidal three-dimensional ion trap with increased ion trapping capacity. Ions are mass-selectively ejected from the ion trap through an elongated aperture corresponding to the elongated storage area.
Though increased ion storage volume is provided by the ion trap geometry of the Bier, et al. patent, the efficiency and versatility of the mass spectrometer suffer, for example, due to the elongated slit and subsequent focusing of the ions required after ejection. In addition, the storage volume is limited by practical considerations, since the length of the spectrometer must be increased in order to increase the ion storage volume.
Space charge effects can also degrade the performance of many mass spectrometers if too many ions are accepted at once for analysis. One solution that has been proposed with limited success is to split the ion current into N independent m/z channels.
There is a need, therefore, to provide an efficient and versatile ion trap, particularly for use in a mass spectrometer, which provides both good ion storage volume and efficient ejection of selected ions.